Electroluminescent (EL) lamps require AC voltage of a certain amplitude and frequency and, therefore, in battery powered equipment, they require a DC to AC inverter.
Unfortunately, however, such inverters require large, relatively costly transformers and many components, or an external signal source, to control the frequency, and this makes them relatively expensive to operate.
Accordingly, there is a need for an inexpensive, small size means by which to power an EL lamp from a single cell inexpensive battery of about 1.5 volts.
Also, in order to power an EL lamp to a minimum of 5 foot-lamberts brightness and, preferably, 10 foot-lamberts brightness, a DC to AC inverter of sufficiently high voltage is needed.
Accordingly, there is need for a system in which a single cell battery can be used to provide an EL lamp of 5-10 foot lambert brightness without having to employ large and expensive transformers.
One example of a system which uses battery means to power EL lamps is the drive circuit described by P. Kindlmann in U.S. Pat. No. 4,527,096.
Kindlmann shows a converter equipped with an inductor for stepping up the battery voltage to successively higher levels to progressively charge the lamp capacitor.
In this system, the charge from the inductor is fed directly into the capacitor and all of the energy is delivered directly to the lamp. An external oscillator means drives the inductor and every pulse out of the inductor is discharged into the capacitor. In this way, the lamp is step-charged until it reaches a predetermined number of successive charging current pulses.
By contrast, the present invention does not depend on current pulses.
Instead in this invention, energy from the inductor is stored in a capacitor, and the stored voltage is used to charge the EL lamp by a constant current.
A stand-alone inverter eliminates the need for an external oscillator or control means for driving the inductor and the energy from the inductor is fed into a storage capacitor to afford what is basically a DC to DC converter of high voltage. This voltage is then used to charge an EL lamp in one direction via a bridge circuit and discharge it the other. In this way, it is possible to generate relatively high DC voltages without damaging the integrated circuit.
As a result, it is now possible to control the current and the frequencies at which EL lamps are driven so that a variety of lamps of various sizes can be operated from one integrated circuit.